1. Field of the Invention
The present invention relates to a terminal unit in an information transmission system including an information transmission line made up of signal lines of positive polarity and negative polarity and, more particularly, to a terminal unit of which plural units are connected to the transmission line and are capable of transmitting and receiving transmission information to the transmission line and from the transmission line in the code of the alternate mark inversion (AMI) mode without being affected by the voltage polarity of the transmission line.
2. Description of the Prior Art
With the recent advancement of electronics, technology in the field of information transmission has also made great progress. The information transmission technology is defined as technology relative to transmission of information that is to be or has been processed by an information processer such as a computer system, and the information transmission system for use in this field of technology in general includes a transmission line connecting the information processors with one another and transmission equipment including input and output terminal units provided between the transmission lines and the information processer. The information transmission technology is not only applied to communications between places of business but recently has come to be used for household communications, such as "home bus system", or communications from outside to home.
FIG. 1A is a block diagram of a first example of a prior art information transmission system as described, for example, in "Reports from Investigation and Research Commission on Household Intelligent System (Final Reports) - Section: Technical Standardization" (published in March 1986, by the Investigation and Research Commission on Household Intelligent System).
FIG. 2 is an explanatory drawing of packet structure and character structure for information transmission and FIG. 3 is an explanatory drawing of the relation between a signal transmitted over a transmission line and a signal delivered from a terminal unit. Referring to FIG. 1A, reference numeral 1 denotes a transmission line consisting of a positive line 1a and a negative line 1b for making power supply and signal transmission, 2 denotes a power supply device consisting of a D.C. power source 2b and an inductance 2a serving as a high impedance to the transmitted signal for supplying power to various units connected to the transmission line 1, and 3 and 13 denote terminal units connected to the transmission line 1. The terminal unit 3 receives power from the transmission line 1 through a power receiving device 4 and makes transmission and reception of signals through a coupling circuit 5. The transmission circuit 8 and reception circuit 11, under the control of a transmission control device 12 exercising control for transmission and reception of signals in accordance with a protocol for the signal transmission, perform transmission to and reception from the transmission line 1 of signals through the coupling circuit 5. The power receiving device 4 is made up of an inductance 4b connected with the positive line 1a of the transmission line 1 for serving as a high impedance to the transmission signal and a stabilizer circuit 4a for voltage stabilization and takes in power from transmission line 1. The coupling circuit 5 consists of capacitors 7a and 7b, whose ends on one side are connected with both terminals of a winding 6a of a transformer 6, including winding 6a, 6b, and 6c, and the ends on the other side are connected with the positive line 1a and negative line 1b of the transmission line 1, respectively. Here, the windings 6b and 6c are connected in series and the middle point thereof is connected with the positive terminal of the power receiving device 4. The transmission circuit 8 includes transistors 9a and 10a, with the collectors of the transistors 9a and 10a connected with one end of the winding 6b and 6c of the transformer 6, respectively, with the emitters connected with ground of the power receiving device 4, and with the bases connected with output terminals 12a and 12b of the transmission control device 12 through resistors 9c and 10c, respectively. Here, bases of the transistors 9a and 10a are connected with the power source positive line of the power receiving device 4 through resistors 9b and 10b, respectively. On the other hand, the reception circuit 11 includes two comparators 11c and 11d, and one input terminal of each of the comparators 11c and 11d is supplied with a reference voltage obtained from divided voltage by resistors 11a and 11b, while the other input terminals are connected with the windings 6b and 6c of the coupling circuit 5, respectively. Output terminals of the comparators 11c and 11d are joined together and connected with the power source positive line of the power receiving device 4 through a resistor 11e and also connected with an input terminal 12c of the transmission control device 12. And, the input terminals of the power receiving device 4 and the capacitors 7a and 7b as input terminals to the coupling circuit 5 are connected with each other and connected with the positive line 1a and negative line 1b of the transmission line 1, respectively. The transmission control device 12, in order to transmit the signal of packet structure and character structure as shown in FIGS. 2(a) and 2(b) in the AMI code, delivers its outputs to the output terminals 12a and 12b, alternately switched therebetween each time the code "0" is transmitted. It may be remarked here that the signal as shown in FIG. 2(a) is structured of priority code PR, self address SA, destination address DA, control code CW, message length BC, data DATA, and check code FCC. Since such a signal is transmitted in an asynchronous manner, the ST bit, or start bit, as shown in FIG. 2(b), is made to be "0" for each terminal unit and the polarity of the transmission line 1 is made to be of the same polarity as that of the supply voltage. The terminal unit 13 is of the same structure as that of the terminal unit 3.
Operation will be described below of the apparatus arranged as above. When the power supply unit 2 is connected to the transmission line 1, the positive line 1a of the transmission line 1 obtains positive polarity and the negative line 1b obtains negative polarity. And, when the terminal unit 3 is connected to the transmission line 1 and a request for transmission is made to the terminal unit 3, the transmission control device 12, after confirming that the transmission line 1 is in an idle state according to a signal from the reception circuit 11, delivers, according to the packet structure as shown in FIG. 2(a), the polarity making the positive line 1a positive. More particularly, it brings its output terminal 12b to the code "1" during the period of start bit ST, whereby a base current flows in the transistor 10a through the resistor 10b, the transistor 10a is turned ON, and the voltage of the power receiving device 4 comes to be applied to the winding 6c so that a voltage with the polarity as indicated by an arrow X shown in FIG. 1A is generated there. Owing to this voltage, a voltage with the polarity in the direction of the arrow X is also generated in the winding 6a, which voltage is delivered through the capacitors 7a and 7b to the transmission line 1 so as to be transmitted over the same superposed on the voltage E supplied from the power supply unit 2 as shown in FIG. 2(b). And, the voltage that is generated in the winding 6b at the time of transmission of the code "0" of the start bit is subjected to level comparison in the comparator 11c, and a voltage as the result of the comparison is applied to the input terminal 12c, whereby the fact that the delivered code is correctly transmitted is verified. When the code "1" is then transmitted, it is achieved by bringing both of the output terminals 12a and 12b to a low level, thereby causing both the transistors 9a and 10a turned OFF. That is, since nothing is output to the transmission line 1, it goes to the voltage level of the power supply unit 2. Further, in the case the code "0" is transmitted next to the transmission of the code "0" of the start bit, the output terminal 12a is brought to a high level, whereby the transistor 9a is turned ON and a voltage with the reverse polarity to that of the start bit is generated so that a voltage with the reverse polarity to that of the supply voltage is applied to the transmission line 1. Thus, serial transmission of an AMI coded baseband signal can be performed. On the other hand, reception of a signal is performed such that the signal transmitted, for example, only by the terminal unit 13 is received by means of the winding 6a through the capacitors 7a and 7b, the level is compared by the comparators 11c, 11d the same as in the case of transmission, and the voltage as the result of the comparison is applied to the input terminal 12c, whereby the transmission control device 12 receives the signal.
Now, the case where the terminal unit 3 and terminal unit 13 started transmission simultaneously will be described. FIG. 3(a) and FIG. 3(b) show signals which the terminal units 3 and 13 desire to transmit, respectively, and FIG. 3(c) shows the signal actually impressed on the transmission line 1.
The code "0" of the start bit transmitted at first is output from each of the terminal units 3 and 13, with the same polarity, and therefore, the output voltage is correctly superposed on the supply voltage in the same polarity so as to be transmitted as the code "0". For the code "1" of the next bit, both the terminal units 3 and 13 also operate equally so as not to superpose any signal on the transmission line 1. And for the code "0" of the next bit, both the terminal units 3 and 13 output a signal of the reverse polarity to that of the start bit, and therefore, the transmitted signal and received signal agree with each other and the transmission is carried out correctly.
In the next bit, the terminal unit 3 tries to transmit the code "0" while the terminal unit 13 tries to transmit the code "1", and as a result, the transmission line 1 comes into a state transmitting the code "0", whereupon since the transmitted signal and received signal are not in agreement for the terminal unit 13, it stops its transmission, while only the terminal unit 3 continues its transmission. Thus, signal transmission using the carrier sense multiple access/collision detection (CSMA/CD) system as the transmission control system can be attained.
In order that the CSMA/CD transmission control is performed for certain, the terminal units 3 and 13 must deliver the code "0" of the start bit in the same polarity to the transmission line 1.
A second example of a prior art information transmission apparatus will be described below.
FIG. 1B is a circuit diagram showing an example of a prior art information transmission apparatus as described, for example, in "Reports from Investigation and Research Commission on Household Intelligent System (Final Reports) - Section: Technical Standardization" (published in March, 1986, by the Investigation and Research Commission on Household Intelligent System.) Referring to the figure, reference numeral 1 denotes an information transmission line (hereinafter to be called the transmission line), and the transmission line 1 consists of a signal line 1a to which the transmitted signal has positive polarity relation and a signal line 1b to which it has negative polarity relation. Reference numerals 3 and 13 denote terminal units connected with each other through the transmission line 1 and transmitting information in an AMI coded baseband signal. The terminal units 3, 13 each include a coupling circuit 5 coupling itself with the transmission line 1, transmission circuit 8, reception circuit 11, and a transmission control device 12.
Construction of each section will be described below in detail. The coupling circuit 5 consists of terminals 51, 52 serving as input terminals, capacitors 7a, 7b connected in series with the terminals 51, 52, respectively, and a coupling transformer having windings 6a, 6b, and 6c.
The transmission circuit 8 consists of transistors 9a, 10a, of which collectors are connected with the windings 6b, 6c of the coupling transformer and emitters are grounded, and base resistors 9c, 10c. The other ends of the resistors 9c, 10c are connected with the transmission control device 12. The AMI code is transmitted by turning the transistors 9a, 10a ON and OFF each time the code "0" is transmitted.
The reception circuit 11 consists of comparators 11c, 11d one input terminal of which is connected with the windings 6b, 6c of the coupling transformer 6 and the other input terminal is connected with a reference voltage generating terminal constituted of a series circuit of resistors 11a, 11b, and an OR circuit 11f having the outputs of the comparators 11c, 11d connected to its input terminals. The output terminal of the OR circuit 11f is connected with input terminal 12c of the transmission control device 12.
Of an AMI code transmitted over the transmission line 1, a signal transmitted with the positive polarity is induced in the winding 6b and subjected to level comparison in the comparator 11c and thereby converted into an NRZ signal. And a transmitted signal of the negative polarity is received by the winding 6c and subjected to level comparison in the comparator 11d and thereby converted into an NRZ signal. Therefore, the reception circuit 11 is a circuit to convert an AMI code into an NRZ code.
The transmission control device 12 is constituted of a microcomputer and the like and includes a start bit transmission terminal 12b for transmitting a signal in positive polarity relation to the signal line 1a, and a transmission terminal 12a for transmitting a signal in negative polarity relation thereto, both the terminals delivering their outputs alternately each time the code "0" is transmitted in order to transmit an AMI code through the transmission circuit 8 to the transmission line 1, and it also includes a reception terminal 12c connected with the output of the reception circuit 11 for receiving a signal transmitted over the transmission line 1.
The information transmission apparatus of the above described organization, while assembling and disassembling the transmission packet as shown in FIG. 2, detects collision by comparing the transmitted data from the transmission terminals 12a, 12b with the data transmitted over the transmission line 1 as shown in FIG. 3, and when it loses in the competition of the collision, it stops its transmission.
Operation will be described below. Upon receipt of a request for transmission, the transmission control device 12 confirms that the transmission line 1 is in an idle state from a signal from the reception terminal 12c, and then, performs data transmission in the AMI code according to the packet structure as shown in FIG. 2(a) from the start bit (ST) for character synchronization. Referring to FIG. 2, PR denotes priority code, SA denotes self address, DA denotes destination address, CW denotes control code, BC denotes message length, DATA denotes data, and FCC denotes check code.
A number of terminal units 3, 13, connected to the transmission line 1, each transmit the code "0" as the start bit to the transmission line 1 to provide the signal line 1a of the transmission line 1 with positive polarity in order that the terminal units 3, 13 detect collision of data in the transmission line 1 from the difference between the data transmitted by the terminal unit 3 (FIG. 3(a)), and that transmitted by the terminal unit 13 (FIG. 3(b)), and the data transmitted, as the result, over the transmission line 1 (FIG. 3 (c)).
That is, the transmission control device 12 keeps its transmission terminal 12b at a high level during a one bit period corresponding to the start bit, whereby the transistor 10a is turned ON and the source voltage is applied to the winding 6c. A voltage corresponding to the source voltage is induced in the winding 6a and applied through the capacitors 7a, 7b to the transmission line 1 with the polarity as shown in the figure, that is, with positive polarity to the signal line 1a.
In transmitting the code "0" in the next place, the transmission terminal 12a is brought to a high level, whereby the transistor 9a is turned ON, reverse voltages to the above are generated in the windings of the coupling transformer 6, and as a result, a signal is transmitted to the transmission line 1 with the polarity in positive relation to the signal line 1b.
And, in transmitting the code "1", both the transmission terminals 12a and 12b are brought to a low level.
In the above cases, the codes "0" and "1" transmitted by turning the transistors 9a and 10a ON/OFF are subjected to level comparison in the comparators 11c, 11d at all times, converted into NRZ signals by the OR circuit 11f, and the transmitted data and the received data of the transmitted data over the transmission line 1 obtained through the reception circuit 11 are compared and detection of collision is performed as shown in FIG. 3.
That is, when the terminal units 3 and 13 try to transmit their signals at the same time, first the codes "0" as the start bit are simultaneously delivered with the same polarity, and therefore, the code "0" is transmitted over the transmission line 1 as shown in FIG. 3(c).
The next code is equally "1" for both the terminal units 3 and 13 and it is transmitted as the code "1" over the transmission line 1. Then, when the terminal unit 3 delivers the code "0" and the terminal unit 13 delivers the code "1", the code "0" is transmitted over the transmission line 1. Therefore, the terminal unit 13 detects a collision from the difference between the delivered data and the received data from the transmission line 1, and hence, it thereafter performs only signal reception.
Now, a third example of the prior art will be described with reference to FIG. 1C.
FIG. 1C is a circuit diagram of a prior art information transmission apparatus as described, for example, in "Reports from Investigation and Research Commission on Household Intelligent System (Final Reports) - Section: Technical Standardization" published in March, 1986, by the Investigation and Research Commission on Household Intelligent System. FIG. 2 is a drawing showing packet structure and character structure for information transmission and FIG. 3 is a time chart indicating the relationship between a signal transmitted over a transmission line and signals delivered from terminal units. Referring to the figure, reference numeral 1 denotes a transmission line for signal transmission consisting of a positive signal line 1a of the positive polarity and a negative signal line 1b of the negative polarity. Reference numerals 3, 13 denote terminal units connected to the transmission line 1 and of the following construction. Reference numeral 12 denotes a transmission control device for transmission and reception of information in accordance with a predetermined protocol, and the same is provided with a transmission data positive output terminal 12a and a transmission data negative output terminal 12b for transmitting signals of the packet structure and character structure as shown in FIGS. 2(a), 2(b) in alternate mark inversion code (AMI code) having redundancy in the direction of the amplitude and alternately delivers its outputs at these output terminals 12a, 12b each time the code "0" is output. Reference numerals 9a, 10a denote transistors, with their respective base terminals connected with the transmission data positive output transistor 12a and the transmission data negative output terminal 12b and form a common-emitter amplifier. Reference numeral 6 denotes a coupling transformer, with windings 6b, 6c connected with the collector terminals of the transistors 9a, 10a, and the center tap of the windings 6a, 6b supplied with the source voltage. Therefore, according to turning ON/OFF of the transistors 9a, 10a, collector-emitter current flows and thereby an AMI coded signal (hereinafter to be called the AMI signal) appears across the winding 6a. Reference numerals 7a, 7b denote capacitors for delivering the AMI signal appearing across the winding 6a of the transformer 6 to the transmission line 1 therethrough. Reference numeral 11 denotes a reception circuit made up of an AMI decoder circuit or the like and is connected with the side of the windings 6b, 6c of the transformer 6. The AMI signal over the transmission line 1 is input by way of the capacitors 7a, 7b and the transformer 6 to the reception circuit 11 made up of an AMI decoder circuit or the like, and the result of the decoding is input to the received signal input terminal 12c of the transmission control device 12.
Operation will be described below. When the terminal unit 3 is connected to the transmission line 1 and a request for transmission is made, for example, to the terminal unit 13, the transmission control device 12, after confirming that the transmission line 1 is in an idle state according to a signal from the reception circuit 11 made up of an AMI decoder circuit or the like, delivers, according to the packet structure as shown in FIG. 2(a), the start bit "0" of each character with the polarity making the positive signal line 1a positive. More particularly, it brings its output terminal 12a to a high level during the period of the start bit, whereby a base current is supplied to the transistor 9a, the transistor 9a is turned ON, and a current flows on the side of the windings 6b, 6c of the transformer 6 from the center tap 6d to the collector of the transistor 9a, so that a voltage of the polarity as indicated by the arrow is generated. This voltage causes a voltage to be induced also on the side of the winding 6a of the transformer 6 as indicated by the arrow, whereby a signal of a waveform as shown in FIG. 2(b) is delivered to the transmission line 1 through the capacitors 7a, 7b. And, the voltage generated in the transformer 6 at the time the code "0" of the start bit is transmitted is also input to the reception circuit 11 made up of an AMI decoder circuit or the like and the result of decoding is read into the received signal input terminal 12c, whereby the fact that the delivered code is correctly transmitted is verified. When the code "1" is then transmitted, it is achieved by bringing both of the output terminals 12a and 12b to a low level, thereby causing both the transistors 9a and 10a to be turned OFF. That is, since nothing is output to the transmission line 1, it exhibits zero voltage. Further, in the case the code "0" is transmitted next to the code "0" of the start bit, the output terminal 12b is brought to a high level, whereby the transistor 10a is turned ON and a voltage with the reverse polarity to that for the start bit is generated, so that a corresponding signal is output to the transmission line 1. Thus, serial transmission of an AMI coded baseband signal is carried out. On the other hand, reception of a signal is performed such that the signal transmitted, for example, only by the terminal unit 13 is received by the transformer 6 through the capacitors 7a, 7b, the signal is decoded in the reception circuit 11 made up of an AMI decoder circuit or the like the same as in the case of transmission, and the decoded signal is input to the received signal input terminal 12c, whereby the transmission control device 12 receives the signal.
Now, the case where the terminal unit 3 and 13 started transmission simultaneously will be described. FIG. 3(a) and FIG. 3(b) show signals which the terminal units 3 and 13 attempt to transmit and FIG. 3(c) shows the signal actually impressed on the transmission line 1. The code "0" of the start bit to be transmitted at first is output with the same polarity by each of the terminal units 3 and 13, and therefore, the same is correctly transmitted over the transmission line 1 as the code "0". For the code "1" of the next bit, both the terminal units 3 and 13 also operate equally so as not to deliver any signal to the transmission line 1. And for the code "0" of the next bit, both the terminal units output a signal of the reverse polarity to that of the start bit, and, when both the terminal units deliver the same code, the transmitted signal and received signal agree with each other, and therefore the transmission is carried out correctly. As to the third bit, the terminal unit 3 tries to transmit the code "0" while the terminal unit 13 tries to transmit the code "1". As a result, the transmission line 1 comes to be transmitting the code "0", whereupon since the transmitted signal and received signal are detected to be in disagreement by the terminal unit 13, it stops its transmission, while the terminal unit 3 continues its transmission since the transmitted signal and received signal are in agreement. Thus, a transmission control system such that, when a collision occurs between a terminal unit 3 and another terminal unit 13, one of them remains undefeated, or "CSMA/CD system with one unit remaining undefeated", can be attained. In order that the "CSMA/CD system with one unit remaining undefeated" is performed for certain, the terminal units 3 and 13 must deliver the start bit code "0" in the same polarity relation to the transmission line 1. If there is a unit connected in reverse polarity relation, the signal level of the transmission line 1 becomes unstable when the start level is transmitted, whereby it occurs that neither of the collided terminal units can confirm agreement between the transmitted and received signals and the condition of the "one unit remaining undefeated" becomes unachievable.
Since the prior art information transmission apparatus have been organized as described above as to their examples, there have been various problems with them as will be mentioned below.
First, in the case of the first example of the apparatus, when signal transmission in the AMI code is made between terminal units connected to the transmission line, if there is a unit connected with the transmission line in the reverse polarity relation thereto, the signal level of the transmission line becomes unstable when the start level is transmitted and the transmission becomes impossible. Thus, there were such problems that the workability was lowered for such reason and trouble causing incapability of transmission was liable to be made by errors in the connection work.
Next, in the case of the second example, to make sure that the collision between signals is correctly detected, the correctness of the polarity of the signal to be impressed on the transmission line becomes essential. Therefore, there were such problems that the polarity of each of the terminal units, when connected to a transmission line, had to be carefully determined; nevertheless, errors in the connection work were liable to be produced.
Finally, in the case of the third example, it is required to confirm the polarity of the terminal units 3, 13 in connecting them to the transmission line 1 in order that disagreement between the transmitted signal to and the received signal from the transmission line 1 is not caused. However, there was such a problem that the terminal units 3, 13 were wrongly connected with the transmission line in their installation work and thereby trouble was caused in the transmission system.